The present invention relates to flat surfaces that emit electrons in localized areas to which an electrical field of threshold magnitude is applied and, in particular, to fabrication of tiny field emitter tips across the surface of a substrate that provides functionality intermediate between thin-film field emitters and field emitter tip microarrays.
The present invention relates to design and manufacture of field emitter tips, including silicon-based field emitter tips. A brief discussion of field emission and the principles of design and operation of field emitter tips is therefore first provided in the following paragraphs, with reference to FIG. 1.
When a wire, filament, or rod of a metallic or semiconductor material is heated, electrons of the material may gain sufficient thermal energy to escape from the material into a vacuum surrounding the material. The electrons acquire sufficient thermal energy to overcome a potential energy barrier that physically constrains the electrons to quantum states localized within the material. The potential energy barrier that constrains electrons to a material can be significantly reduced by applying an electric field to the material. When the applied electric field is relatively strong, electrons may escape from the material by quantum mechanical tunneling through a lowered potential energy barrier. The greater the magnitude of the electrical field applied to the wire, filament, or rod, the greater the current density of emitted electrons perpendicular to the wire, filament, or rod. The magnitude of the electrical field is inversely related to the radius of curvature of the wire, filament, or rod.
FIG. 1 illustrates principles of design and operation of a silicon-based field emitter tip. The field emitter tip 102 rises to a very sharp point 104 from a silicon-substrate cathode 106, or electron source. A localized electric field is applied in the vicinity of the tip by a first anode 108, or electron sink, having a disk-shaped aperture 110 above and around the point 104 of the field emitter tip 102. A second cathode layer 112 is located above the first anode 108, also with a disk-shaped aperture 114 aligned directly above the disk-shaped aperture 110 of the first anode layer 108. This second cathode layer 112 acts as a lens, applying a repulsive electronic field to focus the emitted electrons into a narrow beam. The emitted electrons are accelerated towards a target anode 118, impacting in a small region 120 of the target anode defined by the direction and width of the emitted electron beam 116. Although FIG. 1 illustrates a single field emitter tip, silicon-based field emitter tips are commonly micro-manufactured by microchip fabrication techniques as regular arrays, or grids, of field emitter tips.
Silicon-based field emitter tips can be micro-manufactured by microchip fabrication techniques as regular arrays, or grids, of field emitter tips. Uses for arrays of field emitter tips include computer display devices. FIG. 13 illustrates a computer display device based on field emitter tip arrays. Arrays of silicon-based field emitter tips 1302 are embedded into emitters 1304 arrayed on the surface of a cathode base plate 1306 and are controlled, by selective application of voltage, to emit electrons which are accelerated towards a face plate anode 1308 coated with chemical phosphors. When the emitted electrons impact onto the phosphor, light is produced. In such applications, the individual silicon-based field emitter tips have tip radii on the order of hundreds of Angstroms and emit currents of approximately 10 nanoamperes per tip under applied electrical field strengths of around 50 Volts.
Recently, a second type of field emission display device has been proposed. FIG. 3 illustrates operation of a field emission display device based on a thin-film, flat field emission material. In this alternative type of field emission display device, a semiconductor substrate 302 is coated with a thin film of a material 304 that emits electrons under the influence of a localized electric field. A suitable electric field is created directly below a region of the field emission material 306 with a microelectronic device fabricated within the silicon substrate 308. Electrons emitted from the region of the thin-film field emission material 306 are accelerated in an electric field towards a phosphor-coated glass substrate 312. Collision of an accelerated electron 314 with the phosphor produces phosphorescent emission of photons that travel through the glass substrate 312 to the retina of a viewer. Various research groups have suggested the use of nitrogen-doped, chemical vapor-deposited diamond films, amorphous carbons films, or various conjugated polymers for use as thin-film field emission materials in the flat field emission display devices, operation of which is illustrated in FIG. 3. However, it has proved difficult to fabricate thin-film field emission materials that are long lasting and that produce acceptable current densities of emitted electrons under the influence of reasonably strong electric fields. Thus, designers and manufacturers of field emission display devices have recognized the need for a flat field emission material that can be incorporated in a semiconductor device for use in flat field emission display devices.
The present invention provides a method for fabricating a dense field of tiny, silicon-based field emitter tips across the surface of a silicon substrate. The silicon substrate is first subjected to a beam of oxygen or oxygen-containing ions to create clusters of SiO2 within a thin surface region of the silicon substrate. The clusters of SiO2 molecules created by ionic bombardment of the silicon substrate surface may then be coalesced, if necessary, into clusters by thermal annealing or other techniques. Finally, the surface of the silicon substrate is etched to remove the SiO2 clusters, thereby producing a dense field of tiny silicon-based field emitter tips across the surface of the silicon substrate.